Online/offline detection

I know there is a way to detect whether a player is online or offline.

I want to know what the trigger is that allows a playerto play online or lan even in single player but not play offline

2 systems depending on what you mean exactly, both are conditions

First, this checks if a player is playing or has left

Event - whatever
Cond - player(X) slot status different from (playing) equal to TRUE
Action - kick player(X)

Or, this ends the game if there is only 1 player

Event - whatever
Cond - integer comparison - players in (players matching condition (picked player slot status (different from) (playing) equal to FALSE) equal to 1
Action - group players (pick every players in (all players) and (defeat (picked player)))

It's done with gamecache. You can't read gamecache in multiplayer, so if you set a variable to gamecache and then try to read it, and you get that value, then the map's being played single-player.